1. Field of the Invention
The present invention relates to an optical disc device and a focus control method thereof.
2. Description of the Related Art
When an optical disc is rotated, “surface wobbling” may occur. The “surface wobbling” of the optical disc means that a light irradiation area is moved up and down by rotation of the optical disc.
An optical disc device disclosed in Patent document 1 has a laser, an objective lens, an actuator to drive the objective lens, a focal error detecting means, a spindle motor, a recording means, and a control means. The focal error detecting means detects a focal error of light condensed by the objective lens and outputs an S-shaped focus error signal. The recording means records a wobbling amount (degree of wobbling) when the optical disc is rotated by the spindle motor. The control means performs focus pull-in at a surface of the optical disc when a rotation speed of the optical disc is zero or low enough, and then the rotation speed of the optical disc is increased to record the wobbling amount and controls the actuator after adding the recorded wobbling amount.
An optical disc reproducing device disclosed in Patent document 2 has a light source, an objective lens, a drive means to move the objective lens in an approximately vertical direction with respect to the optical disc, a focus control means, a reproduction means, a recording means, and a control means. The focus control means performs a focus pull-in operation which is an operation of moving the objective lens in a predetermined range by the drive unit so that light from the light source is focused on the optical disc. When the reproduction of the optical disc is interrupted, the recording means records the maximum value and the minimum value of a drive voltage applied to the drive means for making the focus control means drive the objective lens at the time the reproduction is interrupted. When the reproduction of the optical disc is resumed, the control means calculates a frequency of surface wobbling of the optical disc from the maximum value and the minimum value of the drive voltage and the revolution speed of the optical disc at the resume point. The control means determines a drive frequency applied to the drive means based on the frequency of surface wobbling, and controls the focus control means to perform the focus pull-in operation. The drive frequency is corresponding to a moving speed of the objective lens in the focus pull-in operation.
Patent document 1: Japanese Patent Application Laid-Open No. 2008-146714
Patent document 2: Japanese Patent Application Laid-Open No. 2008-21356